In a class with 12 boys and 8 girls, a committee will be chosen with 3 class representatives. The probability of the committee being made up only of girls is:
Probability is number you want/total possible.
Start with total possible: there are 20 people, want committees of 3, that's 20c3, or (20x19x18)/(3x2x1)
Number you want is girls only: 8 people to choose from now, 3 spots, so that's 8c3, or (8x7x6)/(3x2x1)
Put one over the other, cancel out, and you get
56/(20x19x3) whatever that turns out to be.